1. Field of the Invention
This invention relates to semiconductor memory sense circuits particularly to variable threshold transistor memory sense circuits.
2. Description of the Prior Art
Various sense circuits have been described for use with variable threshold transistor memory arrays. One of the simplest sense circuits used in the past is an RS flip-flop. The RS flip-flop is first precharged and then released to select a logic state by the collector or drain which discharges first on one side to latch in one logic state rather than the other. One example of an RS flip-flop used in a non-volatile semiconnector memory is described in U.S. patent application Ser. No. 755,280, filed Dec. 29, 1976, now U.S. Pat. No. 4,090,258, entitled "Improved MNOS Non-Volatile Memory" to J. R. Cricchi and assigned to the assignee herein. In the Patent to Cricchi one side of the RS flip-flop was coupled to a variable threshold transistor of a memory cell written in one memory state while the other side of the flip-flop was coupled to a second variable threshold transistor of the same memory cell written in the opposite direction. A memory cell containing two variable threshold transistors written in the opposite direction relative to one another or detection.
RS flip-flops have also been utilized in memories for detecting the state of a variable threshold transistor where only one variable transistor is used per memory cell by comparing the characteristics of the variable threshold transistor with a second variable threshold transistor written to a predetermined state or a threshold voltage. A variable threshold transistor may also be compared with a known current source or voltage for determining its memory state or threshold voltage. The second transistor or current source or voltage source may have its own variations due to normal manufacturing processes which induce variations during the fabrication of the circuit. In addition, power supply variations may cause variations in the voltage source or current source which is used to be compared with the characteristics of the memory transistor.
The effect of various variations in voltages or currents is to reduce the threshold voltage window or conductance window that differentiates one binary state of a variable threshold transistor from the other. Sense circuits are then required to be more sensitive to narrower differences to detect one binary state from the other.
One approach for a more sensitive sense circuit has been to provide amplification prior to detection by the RS flip-flop or latch. One example of a sense circuit utilizing an amplifier followed by a latch was described by Lawrence G. Heller, D. P. Spampinato and Ying L. Yao in a publication entitled "High-Sensitivity Charge-Transfer Sense Amplifier" in the proceedings of the 1975 IEEE International Solid-State Circuits Conference Digest of Technical Papers, pages 112-113, Feb. 13, 1975. In the publication by Heller et al. the input signal is preamplified using a charge-transfer technique and then fed to an RS flip-flop cross-coupled pair of transistors, for further amplification and storage. The sense amplifier uses eight N-channel transistors exclusive of the memory cell and dummy cell.
It is therefore desirable to provide a novel sense circuit which utilizes a charge-transfer technique for sensitivity in detecting the memory state of a variable threshold transistor in a memory array.
It is further desirable to combine the function of a charge-transfer circuit with a logic circuit to provide a novel sense circuit.
It is desirable to integrate a charge-transfer circuit and a latch to form a novel sense circuit utilizing the minimum number of transistors.